A magnetic resonance imaging apparatus (hereinafter referred to as MRI apparatus) is an apparatus which helps medical diagnosis by radiating a radio-frequency magnetic field pulse to a living body while a uniform static magnetic field is operated, exciting atomic nuclei of hydrogen, phosphorous, or the like in the living body, measuring a nuclear magnetic resonance signal (NMR signal) generated due to the excitation, and imaging a region measured in the living body on the basis of magnetic resonance information such as density distribution and relaxation time distribution of hydrogen or phosphorous.
A large number of methods are applicable to MRI apparatuses in measuring echo signals (NMR signal) as typified by the Spin echo method and the Gradient Echo method. Rapid imaging methods is one of these methods. The method is provided to measure a plurality of echo signals by repeatedly alternating the polarity of a gradient magnetic field in the readout direction after radiating the radio-frequency pulse. For example, the single-shot EPI (Echo Planer Imaging) method, the multi-shot EPI method, and the Gradient and Spin Echo Imaging method are included.
However, with those imaging methods, phase errors of measured echo signals are generated because of not only non-uniformity of the static magnetic field, but also eddy current generated due to rapid alternation of the polarity of the readout gradient magnetic field, and imperfections of the readout gradient magnetic field are generated. Because the polarity of the readout gradient magnetic field is alternately inverted, among all echo signals measured those measured in odd-numbered time (hereinafter referred to as “odd echo”) and those measured in even-numbered time (hereinafter referred to as “even echo”) make opposite trajectories of echo signal collection in the measured space, i.e., the k space, and the polarity of phase error is also alternated in the readout direction.
When phase difference occurs between the odd echo and the even echo as above, in a reconstructed image an artifact described as a N/2 ghost (hereinafter referred to as “N/2 artifact”) is generated. A method of removing the N/2 artifact with signal processing has been proposed, utilizing the fact that the phase error included in the echo signals mainly becomes phase gradient in the readout direction in a real image. This method is designed to acquire data for correction in advance and correct the echo signals in actual measurement for forming an image with the correction data.
As means for this correction, for example, a method is disclosed in Japanese Unexamined Patent Publication. Sho. 8-215174, in which the actual measurement data is corrected by calculating phase difference among echo signals after acquiring as correction data one pair of even echo and odd echo and Fourier-transforming those echo signals. Furthermore, a method is described in Japanese Unexamined Patent Publication. Hei. 5-68674, in which correction data is acquired prior to the actual measurement not applying a phase-encoding gradient magnetic field, the one-dimensional Fourier transformation in the readout direction is performed respectively on this correction data and the actual measurement data, and the phase of the correction data is subtracted from the actual measurement data. Besides, there is a method in which phase difference between an odd echo and an even echo is calculated by using data acquired in two measurements where the polarity of the readout gradient magnetic field is alternated, and thus, the phase error between the odd echo and the even echo in the actual measurement is corrected (“Single-Shot and Segmented EPI Ghost Artifacts Removal with Two-Dimensional Phase Correct”; Nan-kuei Chen, Alice M. Wyrwicz; Proceeding of International Society for Magnetic Resonance in Medicine, 8, 1713 (2000)).
As in the conventional correction method described above, correction of the phase gradient in the readout direction in a real image leads to application of an identical phase correction amount to all pixels in the readout direction in the k space. Accordingly, when a linear or non-linear phase error is generated during sampling of echo signals due to eddy current or imperfections of the gradient magnetic field in the readout direction, the phase error cannot be resolved with the same correction amount and an N/2 artifact is generated. Therefore, generation of artifact in image reconstruction cannot be suppressed.